Previous studies from the Principal Investigator's and other laboratories have convincingly demonstrated a) the mobilization of endothelial progenitor cells (EPC) in response to ischemic stress to the kidney; b) the existence of endogenous mechanisms for EPC mobilization in response to stress - one of those is governed by a surge in uric acid level; c) EPC's participation in renoprotection of ischemic preconditioning; and d) EPC's participation in the eventual repair processes of a post-ischemic organ. These findings infer that the knowledge of intrinsic pathways for EPC mobilization could reveal some novel molecular mechanisms of tissue protection and regeneration, which could be utilized pharmaceutically. We hypothesize that mobilization and proper homing of EPC is not only renoprotective, but also represents a key strategy for induction of pharmacological preconditioning. Specifically, studies will be conducted to establish the pathways leading to EPC mobilization in response to stress: 1) focusing on the metabolites of purine metabolic pathway, especially a reversible post-ischemic surge in uric acid, and cytokines, we shall define the chemical identities of stress (or SOS) signals discharged from the ischemic organ, some of which lead to 2) exocytosis of Weibel-Palade bodies and release of their constituents, such as angiopoietin-2, interleukin- 8 and von Willebrand factor resulting 3) in the systemic mobilization of EPC and engraftment of the affected organ improving its regeneration and, at the same time creating pro-inflammatory conditions. The balance between these two trends will be examined. The possibility of direct or indirect urate signaling via Toll-like receptors will be studied using TLR-2 and -4 knockout mice. Through the use of a stable Lys-Pro-containing peptide which blocks exocytosis of Weibel-Palade bodies in response to ischemia or uric acid, the role of this pathway in post-ischemic injury will be evaluated; this blockade also provides a unique opportunity to select individual components of these organelles which induce maximal pharmacologic preconditioning with minimal pro-inflammatory reaction. Neither of these steps has been previously examined in the context of EPC mobilization. It is anticipated that answers to these questions will define a novel intrinsic pathway(s) for tissue repair involving alarm signaling via post-ischemic surge in uric acid and exocytosis of Weibel-Palade bodies as an effector in EPC mobilization. Refinement of this pathway will be crucial in accomplishing the long-term goal of defining the strategies for pharmacological pre- and post-conditioning. PUBLIC HEALTH RELEVANCE: Kidney injury in response to variety of stressors remains a major problem for health care. Stress- induced mobilization of endothelial progenitor cells (EPC) represents one of the recently discovered intrinsic mechanisms for renoprotection. The study proposed offers the first systematic survey of stress signals emanating from the ischemic kidney to mobilize EPC (focusing on uric acid) and to explore molecular mechanisms governing this process with the long-term goal of designing strategies for pharmacological preconditioning and postconditioning. Furthermore, studies on the functional competence of EPC and their engraftment in diabetic mice subjected to acute kidney injury may provide insights into potential mechanisms of increased predisposition to renal injury in diabetes or metabolic syndrome.